Currently, only Base theme is bundled in core. Since this is responsive, it supports multi device (e.g. Desktop PC, Tablet, Smart phone). And, this theme has some color variations (LightGray, LightBlue, LightCyan, LightIndigo, LightOrange, LightPink, LightTeal, DarkCyan, CoffeeBreak, Social Network Service, University and Custom for you). Click here for details.
Main character of the community is user and contents. So, community site design must be simple without inhibiting main character. Particularly color is important. Many colors will interfere main character. Many of the famous community has been designed along this policy. So, colors of base theme is also simple. Contents (conversation) does not born naturally. It is born from user. Therefore, user is placed at first. And content is placed at side.
Social Network Service: